A problem with wet digestion of organic samples, particularly a gas-generating, tissue digestion such as the Kjeldahl digestion, is boil-over with resultant loss of sample, due to excessive foaming and/or bumping. Further problems are safe disposal of off gases including the corrosive fumes of an acid digesting agent, loss of acid values during digestion, and, for a Kjeldahl digestion especially, length of digestion time.
In recent years, as illustrated by U.S. Pat. No. 4,080,168 to Abu-Samra et al, U.S. Pat. No. 4,347,216 to Kawasaki et al, P. Barrett et al., Analytical Chemistry, 7, 1021 (1978), and a microwave digester made by Prolabo, interest has focused on microwave oven-based wet digestion. However, the problems of small sample size, boil-over, safe disposal of off gases, and loss of acid values exist.
For instance, Abu-Samra et al, while stating that bumping and frothing are virtually eliminated, advise that it may be desirable to incorporate an interrupted duty cycle type of timer to prevent sample boil-over. Kawasaki et al remove off gases through a hollow gas collector connected to an external scrubber and removably mounted, sample decomposing containers; and include an exhaust fan in the microwave oven thereof. However, Kawasaki et al describe a complex equation for selecting an irradiation time and an irradiation interruption period, to control foaming and bumping at an early stage of digestion.
Noting several shortcomings of Abu-Samra et al's fume removal apparatus including deterioration of the interior Plexiglas box, Barrett et al describe a round bottom flask with a ground glass joint connected directly to an exit port which is coupled to an aspirator. However, a ground glass joint in the proximity of a hot, bubbling liquid that may leak into the joint, has a tendency to freeze.
Prolabo's microwave digester vents gases through a container lid connected to a scrubber via a side arm.
As illustrated by R.B. Bradstreet, The Kjeldahl Method for Organic Nitrogen, Academic Press, New York, 1965, pp. 40-42, and a Kjeldahl technique of A/S N Foss Electric, hydrogen peroxide is useful as an additive for reducing boil-over. In the Foss Kjeldahl method, a protein sample is mixed with conventional Kjeldahl digestion ingredients, viz., 10-15 ml concentrated sulfuric acid, 0.75 g mercuric oxide catalyst and 15 g potassium sulfate; 10 ml hydrogen peroxide (35%) is combined with the mixture; the mixture is heated using a high flame; the mixture is heated with a low flame; and 110 ml deionized water is added to the digestate, while cooling the reaction vessel using a blower. Although fast compared to other Kjeldahl methods, this method nevertheless requires twelve minutes to produce a digested sample. The method uses a 0.5 sample when protein content is more than 45%, and a 1.0 g sample when protein content is less than 45%. The apparatus used in the method includes a scrubber connected to a side tube of the reaction vessel.
Also known, as exemplified by a publication authored by S. Brayton of the Hach Company and entitled "A Practical Kjeldahl-Nitrogen Method", is a sulfuric acid-stabilized, hydrogen peroxide prepared by mixing 1 part of concentrated sulfuric acid with 4 parts of 50% hydrogen peroxide. As illustrated by U.S. Pat. No. 3,437,211 to Lindsey and U.S. Pat. No. 4,363,639 to Gladon, a connector tube having coaxial inlet/outlet passageways is known, and as exemplified by U.S. Pat. No. 3,963,420 to Matsumoto et al, sample-dissolving apparatus including a condenser, is known.
Considering the foregoing, it can be understood that there is a need for an improved microwave-based apparatus. Furthermore, there is a need for an improved rapid, microwave-based, macro-Kjeldahl digestion method.